same arguments tailcall bug

[bobzhang]

there is an edge case, when none of the arguments are changed, our optimization will take it as an expression and generate a return statment

let v = ref 0
let acc = ref 0

let rec loop n : int = 
   if !v > n then !acc else begin incr v ; loop n end

A quick fix is to use continue to mark it as a statement, so our engine will not return it, in the future we should have a way to explitly mark Js_output.t terminating...

[bobzhang]

we should also be careful of case break; continue when we do the enhancement later

test case loop_regression_test.ml and format_regression_test.ml